1. Field of the Invention
The invention relates to an energy-selected electron imaging filter including a device for dispersing electrons according to their energies such as a magnetic sector, a device for selecting a small pass-band of electron energies such as a metal slit, several quadrupole and sextupole electron lenses which produce an image with the selected electrons, and an electron imaging device such as a scintillator coupled to a charge-coupled-device camera.
2. Description of Prior Art
Modern electron microscopes are capable of imaging individual atoms in a thin sample, but the images produced by the microscope alone contain no direct information on the chemical composition of the sample, and the image quality degrades significantly if the sample is more than a few atomic planes thick. The chemical information can be provided by selectively imaging only with electrons that have, while traversing the sample, experienced energy losses characteristic of particular atomic species. This means that electrons that have experienced energy losses outside a selected energy pass-band are filtered out, and the energy spread of the electron beam used to form the image is considerably reduced. The image quality, normally degraded by the chromatic aberration of the imaging lenses, is thereby significantly improved.
The energy filtering can be performed in two different ways. The electron beam incident on the sample can be focussed into a narrow probe, which is a raster-scanned across the sample, and the energies of the transmitted electrons can be analyzed at each probe position by an electron energy-loss spectrometer. In this case the energy-filtered image is formed image point by image point, and the recording time for an energy-selected image of 500 by 500 image points is typically around 1 hour. A faster approach is to illuminate the sample with a broad electron beam, and produce an energy-selected image by an apparatus which forms a focussed spectrum of electron energies, selects an energy pass-band, and transforms the spectrum back into an image. Such an apparatus is frequently called an energy-selected electron imaging filter. Attached to a high-performance transmission electron microscope, filters of this type can produce elemental-concentration maps containing a large number of image points in a few seconds. They can also substantially improve the resolution of transmission electron images of thicker samples. This is especially important for biological samples, which can normally only be prepared several hundreds to several thousands of atomic planes thick.
The electron-optical properties of the energy-selected electron imaging filter must be such that the image quality is not noticeably degraded compared to an electron microscope not equipped with such a filter. Prior-art energy-selecting electron imaging filters therefore typically consisted of many independent energy-dispersing and aberration-correcting optical elements, arranged such that large aberations produced by any individual element were cancelled by the combined action of the remaining elements. An example of such a design is described by Rose et al. in Optik vol. 54, pp. 235-250 (1979), hereby incorporated by way of reference. This design comprises four deflecting magnetic sectors, each with specially selected angles and curvatures of entrance and exit faces, three electromagnetic sextupole lenses of adustable strength and polarity, and it further requires at least one round magnetic lens placed after the filter. The design features 7 design parameters which must be independently optimized. The electron path through the filter is relatively complicated, and a small deviation from the ideal path results in a significant degradation of the energy selected image. As a result, this type of energy-selected electron imaging filer has been found very difficult to align. Another complication with this design is that it must be incorporated into the imaging column of the electron microscope. This makes it impossible to install this type of filter as an accessory to an otherwise unmodified electron microscope.
Another example of a design which must be icorporated in the microscope imaging column is the Castaing-Henry filter described by Caistaing et al. in Comptes Rendus d'Academie des Sciences (Paris), vol. 255, pp. 76-78 (1962), hereby also included by way of reference. This design comprises a single magnetic sector, and an electrostatic mirror which causes the electron beam to traverse the sector two times. A major disadvantage of this design is that electrostatic discharges prevent the mirror from operating satisfactorily at electron energies greater than 100 keV, while microscopes most suited to energy-selected imaging typically utilize electrons of energies from 200 keV to 400 keV.
Simple designs of energy-selected electron imaging filters also exist, which make no provision for the correction of image aberrations. However,these designs produce highly distorted and aberrated images whose quality becomes unacceptable for image fields as small as 500 by 500 image points.